Allow libffmpeg to be built for Chromium-based browsers
https://patchwork.ffmpeg.org/patch/4500/

NOTE: This patch lacks the Makefile include line. We append it
manually instead to make patching across different versions easier.

diff --git a/ffbuild/libffmpeg.mak b/ffbuild/libffmpeg.mak
new file mode 100644
index 0000000..992cf3c
--- /dev/null
+++ b/ffbuild/libffmpeg.mak
@@ -0,0 +1,21 @@
+LIBFFMPEG = $(SLIBPREF)ffmpeg$(SLIBSUF)
+LIBFFMPEG_LINK = $(LD) -shared -Wl,-soname,$(LIBFFMPEG) -Wl,-Bsymbolic -Wl,-z,now -Wl,-z,relro -Wl,-z,defs -Wl,--gc-sections $(LDFLAGS) $(LDLIBFLAGS) -o $(LIBFFMPEG)
+
+libffmpeg-: libavcodec/$(LIBPREF)avcodec$(LIBSUF) libavformat/$(LIBPREF)avformat$(LIBSUF) libavutil/$(LIBPREF)avutil$(LIBSUF) libswresample/$(LIBPREF)swresample$(LIBSUF)
+	$(LIBFFMPEG_LINK) -Wl,--whole-archive $^ -Wl,--no-whole-archive $(FFEXTRALIBS)
+
+libffmpeg-yes: libavcodec/$(SLIBPREF)avcodec$(SLIBSUF) libavformat/$(SLIBPREF)avformat$(SLIBSUF) libavutil/$(SLIBPREF)avutil$(SLIBSUF)
+	$(LIBFFMPEG_LINK) -Wl,--no-as-needed -lavcodec -lavformat -lavutil
+
+$(LIBFFMPEG): libffmpeg-$(CONFIG_SHARED)
+libffmpeg: $(LIBFFMPEG)
+
+install-libffmpeg: $(LIBFFMPEG)
+	$(Q)mkdir -p "$(SHLIBDIR)/chromium"
+	$(INSTALL) -m 755 $< "$(SHLIBDIR)/chromium/$<"
+	$(STRIP) "$(SHLIBDIR)/chromium/$<"
+
+uninstall-libffmpeg:
+	$(RM) "$(SHLIBDIR)/chromium/$(LIBFFMPEG)"
+
+.PHONY: libffmpeg libffmpeg-* install-libffmpeg
-- 
2.13.1